Microfabricated structures for facilitating fluid introduction into microfluidic devices

ABSTRACT

Fluid introduction is facilitated through the use of a port which extends entirely through a microfluidic substrate. Capillary forces can be used to retain the fluid within the port, and a series of samples or other fluids may be introduced through a single port by sequentially blowing the fluid out through the substrate and replacing the removed fluid with an alternate fluid, or by displacing the fluid in part with additional fluid. In another aspect, microfluidic substrates have channels which varying in cross-sectional dimension so that capillary action spreads a fluid only within a limited portion of the channel network. In yet another aspect, the introduction ports may include a multiplicity of very small channels leading from the port to a fluid channel, so as to filter out particles or other contaminants which might otherwise block the channel at the junction between the channel and the introduction port.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of, and claims the benefit of priorityfrom, co-pending U.S. patent application Ser. No. 09/ 539,671, filedMar. 30, 2000, now U.S. Pat. No. 6,451,188 which is a divisional of U.S.patent application Ser. No. 08/870,944, filed Jun. 6, 1997, now U.S.Pat. No. 6,090,251, the full disclosures of which are incorporatedherein by reference.

This application is also related to U.S. patent application Ser. No.09/274,811 filed Mar. 23, 1999, the full disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to microfluidic systems anddevices and methods for their use. More particularly, the presentinvention provides structures and methods which facilitate theintroduction of fluids into devices having microfluidic channels.

Considerable work is now underway to develop “microfluidic” systems,particularly for performing chemical, clinical, and environmentalanalysis of chemical and biological specimens. The term microfluidicrefers to a system or device having a network of chambers connected bychannels, in which the channels have mesoscale dimensions, e.g., havingat least one cross-sectional dimension in the range from about 0.1 μm toabout 500 μm. Microfluidic substrates are often fabricated usingphotolithography, wet chemical etching, and other techniques similar tothose employed in the semiconductor industry. The resulting devices canbe used to perform a variety of sophisticated chemical and biologicalanalytical techniques.

Microfluidic analytical systems have a number of advantages overconventional chemical or physical laboratory techniques. For example,microfluidic systems are particularly well adapted for analyzing smallsample sizes, typically making use of samples on the order of nanolitersand even picoliters. The substrates may be produced at relatively lowcost, and the channels can be arranged to perform numerous specificanalytical operations, including mixing, dispensing, valving, reactions,detections, electrophoresis, and the like. The analytical capabilitiesof microfluidic systems are generally enhanced by increasing the numberand complexity of network channels, reaction chambers, and the like.

Substantial advances have recently been made in the general areas offlow control and physical interactions between the samples and thesupporting analytical structures. Flow control management may make useof a variety of mechanisms, including the patterned application ofvoltage, current, or electrical power to the substrate (for example, toinduce and/or control electrokinetic flow or electrophoreticseparations). Alternatively, fluid flows may be induced mechanicallythrough the application of differential pressure, acoustic energy, orthe like. Selective heating, cooling, exposure to light or otherradiation, or other inputs may be provided at selected locationsdistributed about the substrate to promote the desired chemical and/orbiological interactions. Similarly, measurements of light or otheremissions, electrical/electrochemical signals, and pH may be taken fromthe substrate to provide analytical results. As work has progressed ineach of these areas, the channel size has gradually decreased while thechannel network has increased in complexity, significantly enhancing theoverall capabilities of microfluidic systems.

Unfortunately, work in connection with the present invention has foundthat the structures and methods used to introduce samples and otherfluids into microfluidic substrates can limit the capabilities of knownmicrofluidic systems. Fluid introduction ports provide an interfacebetween the surrounding world and the microfluidic channel network. Thetotal number of samples and other fluids which can be processed on amicrofluidic substrate is now limited by the size and/or the number ofports through which these fluids are introduced to the microfluidicsystem. Known structures and methods for introduction of fluids intomicrofluidic systems also generally result in the transfer of a muchgreater volume of fluid than is needed for microfluidic analysis.

Work in connection with the present invention has also identifiedunexpected failure modes associated with known methods for introducingfluids to microfluidic channels. These failure modes may result in lessthan desirable overall reliability for microfluidic systems. Finally, aneed has been identified for some mechanism to accurately pre-positiondifferent fluids within a contiguous microfluidic network, so as tofacilitate a variety of microfluidic analyses.

It would therefore be desirable to provide improved structures, systems,and methods which overcome or substantially mitigate at least some ofthe problems set forth above. In particular it would be desirable toprovide microfluidic systems which facilitated the transfer of smallvolumes of fluids to an introduction port of a microfluidic substrate,and to increase the number of fluids which can be manipulated within thesubstrate without increasing the overall size of the substrate itself.It would be particularly desirable to provide microfluidic introductionports which could accept multiple fluid samples, and which were lessprone to failure than known introduction port structures. Finally, itwould be advantageous to provide microfluidic channel networks which areadapted to controllably pre-position differing liquids within adjoiningchannels for analysis of samples using differing fluid media.

SUMMARY OF THE INVENTION

The present invention overcomes at least some of the deficiencies ofknown structures and methods for introducing fluids into microfluidicsubstrates. In some embodiments, fluid introduction can be facilitatedthrough the use of a port which extends entirely through the substratestructure. Capillary forces can be used to retain the fluid within sucha through-hole port, rather than relying on gravity to hold the fluidwithin a cup-like blind hole. A series of samples or other fluids may beintroduced through a single through-hole port by sequentially blowingthe fluid out of the port, and replacing the removed fluid withdifferent fluid. Advantageously, an array of such through-hole ports canwick fluids from the surfaces of a corresponding array of pins, therebyavoiding the need for complex pipette systems. In another aspect, thepresent invention provides microfluidic substrates having channels whichvary in cross-sectional dimension so that capillary action spreads afluid only within a limited portion of the channel network. In yetanother aspect, the introduction ports of the present invention mayinclude a multiplicity of very small channels leading from the port to alarger microfluidic fluid channel. These small channels filter outparticles or other contaminants which might other vise block themicrofluidic channel.

In a first aspect, the present invention provides a microfluidic systemcomprising a substrate having an upper surface, a lower surface, and amicrofluidic channel disposed between these surfaces. A wall of thesubstrate borders a port for receiving fluid. The port is in fluidcommunication with the channel, and the port is open at both the uppersurface of the substrate, and at the lower surface of the substrate.

Generally, the port has a cross-sectional dimension which issufficiently small so that capillary forces restrain the fluid withinthe port. The specific size of the port will depend in part on theproperties of the material along its border. The capillary forcesbetween the port and the fluid can also be used to transfer the fluidfrom the outer surface of a pin, rather than relying on a complexpipette system. The use of a through-hole port also facilitates theremoval of the fluid from the port, as the fluid can be blown throughthe substrate with differential pressure, or simply displaced from theport with an alternate fluid. Optionally, the lower surface of thesubstrate may have a hydrophobic material to prevent the sample fromspreading along the lower surface, while a hydrophilic rod or capillarytube may facilitate decanting of the fluid from the port.

In another aspect, the present invention provides a method forintroducing a fluid into a microfluidic channel of a substrate. Themethod comprises transporting the fluid from outside the substrate to aport of the substrate through a first surface. The port extends throughthe substrate, and opens on a second surface of the substrate. Themicrofluidic channel of the substrate is in fluid communication with theport between the first and second surfaces. The fluid is restrainedwithin the port at least in part by a capillary force between the portand the fluid.

In yet another aspect, the present invention provides a method forintroducing a plurality of samples into a microfluidic substrate. Themethod comprises forming a volume of each sample on an associated pin.The pins are arranged in an array, and the array of pins is aligned withan array of ports on the substrate. The aligned pins and ports arebrought together so that the volumes transfer from the pins toassociated ports of the substrate.

In yet another aspect, the present invention provides a method forintroducing a plurality of fluids into a microfluidic substrate. Themethod comprises inserting a first fluid into a port of the substrate. Aportion of the first fluid is transferred from the port into amicrofluidic channel of the substrate. An unused portion of the firstfluid is removed from the port, and a second fluid is inserted into theport.

The present invention also provides a microfluidic system comprising abody having a first channel and a capillary limit region. A secondchannel is in fluid communication with the first channel through thelimit region. The second channel has a cross-sectional dimensionadjacent the limit region which is larger than a cross-sectionaldimension of the limit region. This difference in cross-sectionaldimensions inhibits wicking from the limit region into the secondchannel.

Generally, a minimum cross-sectional dimension of the limit region issufficiently smaller than a minimum cross-sectional dimension of thesecond channel so that differential capillary forces prevent wicking offluid from the first channel, through the limit region, and into thesecond channel when there is no fluid in the second channel. Typically,the first channel and limit region end at the intersection with thesecond channel, while the second channel continues on past theintersection (like the top bar in a “T”). This structure is particularlyadvantageous to establish predetermined boundaries between two differentfluids within a microfluidic channel network, as a fluid which isintroduced into the first channel will wick through the channel to thelimit region, but will not wick beyond the limit region into the secondchannel. A second different fluid can then wick through the secondchannel, beyond the intersection with the first limit region, therebydefining a boundary between the first and second fluids at the channelintersection.

In another aspect, the present invention provides a method forcontrollably distributing fluids within microfluidic substrates. Themethod comprises wicking a first fluid along a first channel and into acapillary limit region. The first fluid is prevented from wicking beyondthe limit region and into a second channel by differential capillaryforce.

The present invention also provides a filtered microfluidic systemcomprising a substrate having a reservoir and a channel having a fluidmicrofluidic cross-section. A plurality of filter channels extend inparallel between the reservoir and the channel. Each filter channel hasa cross-sectional dimension which is smaller than a fluid channelcross-sectional dimension of the microfluidic channel.

In yet another aspect, the present invention provides a method forfiltering a fluid sample entering a microfluidic channel network. Themethod comprises introducing the fluid sample into a port, and passingthe fluid sample through a plurality of filter channels which arearranged in parallel. The filter channels block particles havingcross-sections which are larger than a maximum filter particle size. Thefiltered fluid sample is collected and transported through amicrofluidic channel having a cross-section which is larger than themaximum filter size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical microfluidic fluidintroduction system, in which a pipette deposits fluid in a blind hole.and in which the fluid must pass through a single microfluidic channelto enter the channel network.

FIG. 2 is a perspective view in partial cross-section showing a systemfor introducing an array of fluid samples to a corresponding array ofthrough-hole ports, and also shows the use of hydrophilic rods tofacilitate decanting the fluid samples from the through-hole ports.according to the principles of the present invention.

FIG. 3 is a cross-sectional view illustrating the use of capillaryforces to retain a fluid sample within a through-hole port, and alsoillustrates the use of electrokinetic forces to transport the fluidwithin the microfluidic substrate.

FIG. 4 is a cross-sectional view showing the use of differentialpressure and a hydrophilic rod to decant a sample from a through-holeport.

FIG. 5 is a plan view of an integrated reservoir and filter to preventparticles from blocking the microfluidic channels of the substrate.

FIG. 6 is a cross-sectional view showing the integrated port and filterof FIG. 5.

FIG. 7 schematically illustrates a microfluidic substrate having fluidstops which allow two different fluids to be positioned within thenetwork, with the boundaries between the fluids being located atpredetermined limit regions.

FIGS. 8 and 9 are cross-sectional views showing the structure andoperation of the fluid stop limit regions of FIG. 7.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

A typical microfluidic introduction system and method is schematicallyillustrated in FIG. 1. A substrate 10 generally comprises an upperportion 12 through which a port 14 has been drilled. A lower portion 16is bonded to upper portion 12, the lower portion having a microfluidicchannel 18 which is in fluid communication with port 14. A pipette 20delivers fluid 22 to port 14, typically relying on pneumatic and/orhydraulic pressure to deposit the fluid in the port.

Work in connection with the present invention has identified failuremodes which could prevent fluid 22 from reaching channel 18, therebyinterfering with the intended operation of microfluidic substrate 10. Inthe first failure mode, any particles in the fluid, in the pipette, orin the port may flow with the fluid from the port toward channel 18.Particles which are not large enough to enter microfluidic channel 18will be deposited at channel entrance 24, thereby blocking flow from theport to the channel. As microfluidic channels get smaller and smaller.there is a corresponding increase in sensitivity to even minuteparticles of contamination blocking the entrance 24 to port 18.

In another failure mode for typical microfluidic structures, the dropsdeposited by pipette 20 into port 14 may include bubbles, or air (orother gases) may be trapped within the port below the drop of fluid.Where an air bubble covers entrance 24 to port 18, the fluid will notenter the channel through capillary wicking.

As the advantages of microfluidic structures are generally enhanced bydecreasing the size of the system components, it is generally desirableto decrease the size of port 14. For example, this allows thefabrication of microfluidic systems having larger numbers of fluid portson a substrate of a given size. This would allow each substrate tosimultaneously analyze larger numbers of samples, or may alternativelyallow more complex chemical or biochemical analyses to be performed.Regardless, as the size of port 14 decreases the likelihood that abubble will be trapped under the fluid increases. In fact, port 14 mayeventually be made small enough that fluid remains over the uppersurface of the substrate without substantially entering port 14.

To overcome these failure modes and disadvantages, microfluidic fluidintroduction system 30 includes a microfluidic substrate 32 having anarray of through-hole ports 34, as illustrated in FIG. 2. Samples andother fluids are transferred into through-hole ports 34 as drops 36 onthe outer surfaces of a corresponding array of pins 38. Surprisingly,through-hole ports 34 extend entirely through substrate 32 from an uppersurface 40 to a lower surface 42. Drops 36 will wick into through-holeports 34, and will be restrained within the through-hole ports bycapillary forces between the fluid and the surrounding ports. A fluidremoval system 44 includes rods 46 which facilitate decanting the fluidfrom the through-hole ports, as will be described in more detailhereinbelow.

Pins 38 are mounted on a pin support structure 48. As pins 38 arealigned with through-hole ports 34, a large number of individual drops36 may be transferred simultaneously from the pins to the through-holeports by moving pin support structure 48 into close proximity withsubstrate 32. Drops 36 may be formed on pins 38 by dipping the pins inan associated array of fluid receptacles, by distributing the fluidthrough channels within fluid support structure 48, or the like. As onlyvery small amounts of fluid are needed for the microfluidic analysis,the size of drops 36 can be quite small. By relying on pins to transferdrops on their outer surfaces (rather than individual pipettes withcomplex hydraulic or pneumatic systems), the cost and complexity of asystem for transporting a large number of discrete drops of fluid intoassociated microfluidic ports can be substantially reduced. The pins mayoptionally be aligned in an array corresponding to at least a portion ofa standard microtiter plate, e.g,. 12 rows of 8 pins on 9 mm spacings,to facilitate preparing samples and other fluids with conventionalchemical and biological techniques.

As drops 36 enter through-hole ports 34, they are drawn into the portsby both gravity and capillary forces. As through-hole ports 34 extendentirely through substrate 40, no air can be trapped between the dropsand the bottom of the port. As the through-hole ports rely on capillaryforces to retain the fluid, it should be noted that the orientation ofthe port can be changed from vertical to horizontal, angled, etc., sothat the terms “upper surface” and “lower surface” are relative to anarbitrary orientation of the substrate. Nonetheless, an at leastpartially vertical orientation may be preferred to facilitatetransferring drops 36 on pins 38 to through-hole ports 34.

Generally, capillary forces draw fluids from larger channels to smallerchannels. More specifically, capillary forces are largely controlled bythe minimum cross-sectional dimension of a channel. For example,capillary forces will wick a fluid from a channel having a width of 100micrometers and a depth of 20 micrometers into a contiguous channelhaving a width of 100 micrometers and a depth of 10 micrometers. Hence,simple capillary forces may optionally be relied on to draw fluid fromthrough-hole port 34 into microfluidic channels within substrate 32 (notshown in FIG. 2), so long as the microfluidic channels have a smallercross-sectional dimension than the smallest cross-sectional dimension ofthe through-hole ports. Additional or alternative mechanisms are alsoavailable for injecting fluid from the through-hole ports into themicrofluidic channels of the substrate. including electrokinetics,differential pneumatic pressure, and the like.

As can be understood with reference to FIG. 3. application of anelectrical current, potential, or charge between microfluidic channel 48and a fluid 50 within through-hole port 34 can help inject the fluidinto the channel. Typically, an electrical power source 52 will becoupled to a waste fluid reservoir electrode 54, and to a port electrode56 (and/or pin 38). Port electrode 56 is coupled to fluid 50 through anelectrical access port 57. The port access electrode and waste portelectrode may be formed as conductors which extend downward into theirassociated ports from pin support structure 48, or from a separateelectrical connector assembly, so that no electrodes need beincorporated into substrate 32. As used therein the term portencompasses the structure of a microfluid substrate which allows accessto the microfluidic channels for introducing fluids and other materials,and/or for electrically coupling electrodes to the fluid within thechannels. The term reservoir encompasses ports and other structures ofthe substrate which accommodate a significantly greater volume of fluidthan the microfluidic channels. The use of electrokinetics as atransportation mechanism within microfluidic channels is more fullydescribed in U.S. Pat. No. 5,880,071, and in Published PCT ApplicationNo. WO 96/04547, the full disclosures of which are incorporated hereinby reference. Similar transportation mechanisms may facilitate transferof the fluid from the outer surface of pin 38 to through-hole port 34 bythe application of an electrical field through the pin and portelectrode 56. Alternatively, the through-hole ports of the presentinvention are also well suited for use with standard pipette systems.

Useful substrate materials include glass, quartz and silicon, as well aspolymeric substrates, e.g., plastics. In the case of polymericsubstrates, the substrate materials may be rigid, semi-rigid, ornon-rigid, opaque, semi-opaque or transparent, depending upon the usefor which they are intended. For example, devices which include anoptical or visual detection element, will generally be fabricated, atleast in part, from transparent materials to allow, or at leastfacilitate that detection. Alternatively, transparent windows of, e.g.,glass or quartz, may be incorporated into the device for these types ofdetection elements. Additionally, the polymeric materials may havelinear or branched backbones, and may be crosslinked or non-crosslinked.Examples of particularly preferred polymeric materials include, e.g.,polymethylmethacrylate (PMMA) polydimethylsiloxanes (PDMS),polyurethane, polyvinylchloride (PVC), polystyrene, polysulfone,polycarbonate, and the like.

The cross-sectional dimensions of through-hole port 34 will typically beselected to provide sufficient capillary force between fluid 50 and theport to at least help restrain the fluid within the port. Preferably,the cross-section will have a minimum diameter which is sufficient toinduce a capillary force which will overcome the force of gravity (whichpulls fluid 50 through the open bottom of the through-hole port). Thespecific minimum cross-sectional dimensions of through-hole port 34which will provide this capillary force will depend on the wetability ofthe material bordering the port, the fluid to be retained therein, thedistance between the channel and the bottom of the substrate if thethrough-hole port has a vertical orientation, and the like. For example,through-hole ports in many plastic materials will be smaller thansimilar through-hole port structures in glass substrates, due to thehigher wetability of glass.

Through-hole ports 34 will typically be drilled through substrate 32with a circular cross-section, the cross-section of the through-holeport typically having a diameter of between about 0.1 mm and 5 mm, andideally having a diameter within the range of from about 0.5 mm to 2 mm.Such holes may be drilled using “air abrasion”, an erosion process whichis similar to a precisely directed sandblast of the substrate material.Air abrasion services are commercially available from NYS Enterprises ofPalo Alto, Calif. Alternatively, ultrasonic drilling or laserphotoablation may be used to provide quite small ports through thesubstrate. In other embodiments, small carbide drill bits maymechanically drill thorough the substrate to provide through-hole portshaving small enough cross-sectional dimensions to induce the desiredcapillary forces. Through-hole ports may also be formed during thesubstrate molding or embossing processes, particularly when thesubstrates comprise polymeric materials.

While the structures are here illustrated as having slightly taperingcross-sections, they may alternatively have constant diameters, or maydecrease near one or both surfaces. The holes may be drilled through theentire substrate in one operation, or may alternatively be drilledindependently through separate upper and lower portions of the substrateprior to bonding these portions together. The cross-section of thethrough-hole ports need not be the same through the upper and lowerportions, and should be tolerant of some mismatch between the locationand size of the openings formed in the upper and lower portions of thesubstrate. A wide variety of alternative port cross-sectional shapes mayalso be used, with the diameter ranges given above generally definingthe minimum cross-sectional dimension. For example rectangular (or anyother arbitrary shape) ports may be formed in at least one portion ofthe substrate structure while the channels are formed by etching afenestration through the substrate portion.

Regardless of the specific cross-section, the through-hole ports willpreferably have a total volume between the upper and lower surfaces ofthe substrate of less than about 20 μl, ideally having a volume ofbetween about 0.5 μl and 10 μl. As the through-hole ports of the presentinvention generally facilitate the use of smaller sample volumes, theyare particularly advantageous for use in drug discovery applications,such as those described in co-pending U.S. Pat. No. 6,046,056, the fulldisclosure of which is incorporated herein by reference.

Referring now to FIG. 4, a particular advantage of through-hole ports 34is that they facilitate the introduction of multiple fluids into amicrofluidic network using a single port structure. Fluid 50 may beremoved from through-hole port 34 by applying a differential gaspressure P over the top of substrate 32 (relative to the pressure belowthe substrate), effectively blowing the fluid out through thethrough-hole port. Optionally, rods 46 decant fluid 50 from thethrough-hole port when the pressure extends the fluid more than adistance D beyond lower surface 42. A hydrophobic coating 58 (e.g., apolytetrafluoroethylene such as Teflon™ helps prevent smearing of fluid50 over lower surface 42 of substrate 32, thereby avoidingcross-contamination of fluid samples. Decanting may be enhanced by ahydrophilic coating 60 on the surface of rod 46, or alternatively byusing decanting structures which have a capillary channel. Fluid removedfrom through-hole port 34 is collected in well 62, and the wells mayoptionally be connected by drains to a fluid disposal system.

While differential pressure is a particularly advantageous mechanism forsimultaneously removing fluids from multiple through-hole ports in asubstrate, the present invention also encompasses other mechanisms forsimultaneously or individually removing the samples, includingelectrokinetically distending the sample from lower surface 42 (as canbe understood with reference to FIG. 3), displacing fluid 50 with analternate fluid introduced into ports 34 through upper surface 40 (usinga pipette, pins 38, or the like), inserting decanting structures intoports 34, and the like. In general, fluid 50 may be directly replaced byan alternate fluid for use in the fluidic network, or a cleaning orneutral solution may first be entered into through-hole port 34 tominimize cross-contamination of the sequentially introduced fluids.Regardless, the ability to sequentially introduce multiple fluids into amicrofluidic network through a single port substantially enhances theeffectiveness of that port as an interface between the microfluidicnetwork and the surrounding world.

Referring now to FIGS. 5 and 6, a filtered port 64 in substrate 32 isillustrated with a blind reservoir 66, but may alternatively be usedwith the through-hole port structure described hereinabove. Reservoir 66is defined by a hole 68 drilled through upper portion 12 of substrate32, while a microfluidic channel 18 has been imposed on lower portion16. To prevent particles from blocking the entry to channel 18, amultiplicity of radial filter channels 70 lead from reservoir 66. Filterchannels 70 transmit fluid from reservoir 66 to a header channel 72which in turn opens to channel 18. However, particles larger than somemaximum filter particle size (which will vary with the cross-section ofthe filter channel) will be left in the port. This prevents largeparticles from blocking channel 18.

Filter channel 70 has at least one smaller cross-sectional dimensionthan channel 18, the filter channel often being smaller incross-sectional area than channel 18. Preferably, the filter channels 70are individually sufficiently small to block entry of particulates whichmight impede flow through channel 18. However, there are a sufficientnumber of functionally parallel filter channels so that the sum of thecross-sectional areas of all the filter channels together is at least aslarge as channel 18, ideally being substantially larger than channel 18to minimize head loss through the filter structure. In fact, as filterchannels 70 may individually be blocked by particulates, the sum of thecross-sectional areas of the filter channels will determine the filtercapacity. In other words, the more total cross-sectional area of filterchannels, the more particulate matter the filter can remove from theflow before the filter becomes blocked. Hence, the total cross-sectionalarea of all the filter channels together will preferably be in the rangefrom about 2 to about 100 times larger than the cross-section of channel18. Header channel 72 will typically be about the same size as channel18.

Channel 18 will typically have a minimum cross-sectional dimension ofbetween about 0.5 and 100 μm. Filter channels 70 will generally besmaller than fluid channel 18, ideally having a minimum cross-sectionaldimension of between about 10 and 50% of the minimum cross-sectionaldimension of channel 18. There will generally be between about 10 and100 functionally parallel filter channels. Typical channel dimensionsare about 10 micrometers deep and 70 micrometers wide for channel 18 andheader channel 72, while the corresponding filter channels willtypically be about 2 micrometers deep and 10 micrometers wide.

A wide variety of reservoir, filter channel, and header channelgeometries might be used to prevent blockage of fluids as they enterfluid channel 18. For example, filter channels 70 may extendgeometrically parallel to each other from one side of reservoir 68 to astraight header channel normal to fluid channel 18. However, the radialfilter geometry illustrated in FIG. 5 is preferred, as it minimizes thesubstrate surface area consumed by the filter.

Referring now to FIGS. 7-9, it will be useful in many microfluidicnetworks to pre-position different fluids within a microfluidic networkat predetermined locations. For example, a microfluidic channel network74 includes an electroosmotic channel 76 from which threeelectrophoretic separation channels 78 extend. Electrophoretic channels78 will preferably contain a separation solution including a polymer,while electroosmotic channel 76 will preferably be filled with a buffersolution to facilitate transportation of a fluid sample from filteredreservoir 64. Unfortunately, if all of the channels have uniformcross-sections, any fluid introduced into any of the reservoirs 64, 80,82, or 84, will wick throughout channel network 74.

To limit the capillary wicking of a first solution 86 to electrophoreticchannels 78, the first solution is introduced into one of the adjoiningreservoirs 82, 84. First solution 78, which will be an electrophoreticpolymer containing solution in our example, will wick along across-channel 88 and into each of electrophoretic channels 78.Furthermore, the first solution will wick along each of theelectrophoretic channels toward electroosmotic channel 76. The airdisplaced from within the electrophoretic channels can escape throughelectroosmotic channel, and out through the adjoining ports.

To prevent the first fluid from filling the electroosmotic channel 76, alimit region 90 is disposed adjacent the junction of the two types ofchannels. Limit region 90 will have at least one cross-sectionaldimension which is smaller than a cross-sectional dimension of theadjacent electroosmotic channel 76, the limit region ideally having anarrowest cross-sectional dimension which is smaller than the narrowestcross-sectional dimension of the electroosmotic channel. As a result,the first fluid will wick in to the limit region from electrophoreticchannel 78, but differential capillary forces will prevent first fluid86 from passing through limit region 90 and wicking into electroosmoticchannel 76. The ratio of the minimum cross-sectional dimensions mayagain vary with the properties of the materials bordering the limitregion and channels, with the limit region generally having a minimumdimension of less than 90% that of the channel. Typical electroosmoticand electrophoretic channel dimensions will be about 70 μm wide by 10 μmdeep, while the corresponding limit regions may be about 70 μm wide byabout 2 μm deep.

A second fluid 92 introduced at reservoir 80 will wick throughelectroosmotic channel 76 past limit regions 90, thereby defining aninterfluid boundary 94 substantially disposed at the interface betweenlimit region 90 and electroosmotic channel 76. It should be noted thatelectroosmotic channel extends across limit regions 90 (rather thanhaving a dead end at the limit region) to avoid trapping air betweenfirst fluid 86 and second fluid 92. As a result, the air withinelectroosmotic channel 76 is free to leave the opening provided atfiltered reservoir 64, so that all of the channels of channel network 74are substantially filled with fluid. Although this example has beendescribed in terms of “electrophoretic” and “electroosmotic” channels,it will be appreciated that the present invention can be used in anyapplication where it may be desirable to place different fluids withinintersecting channel structures.

It should also be noted that second fluid 92, will wick into headerchannel 72 so long as the header channel is not significantly larger inits narrowest cross-sectional dimension than electroosmotic channel 76.Additionally, the buffer solution will proceed into the small filterchannels 70 from header channel 72. However, the buffer solution willgenerally not advance beyond filter channels 70 into reservoir 66, asthe filter channels effectively provide limit regions between thereservoir and the header channel. To prevent this “limit region” effectof the filter channels from inhibiting flow from the reservoir into theadjacent channel system, it will generally be preferable to introducesome fluid into the header and filter channels prior to introducing afluid directly into reservoir 66. Similarly, fluid channel networkshaving a plurality of fluid introduction ports will generally include atleast one unfiltered port structure. Otherwise, it might be difficult toadvance any fluid into the network beyond the small filter channelssurrounding each port.

While the exemplary embodiments of the present invention have beendescribed in some detail, by way of illustration and for clarity ofunderstanding, a number of modifications, adaptations, and alternativeembodiments will be obvious to those of skill in the art. For example,the present invention may be used with microfluidic structures that relyon pneumatic pressure or a vacuum to move materials within microfluidicchannels. Therefore, the scope of the present invention is limitedsolely by the appended claims.

What is claimed is:
 1. A microfluidic device comprising: a substratehaving a plurality of first microfluidic channels and at least onesecond microfluidic channel, said plurality of first channels being influid communication with the at least one second channel through arespective capillary limit region, the second channel including a firstend and a second end, wherein the limit regions are disposed at an endof each respective first channel and intersect the second channelbetween its first and second ends; and the second channel having across-sectional dimension adjacent the limit regions which is largerthan a cross-sectional dimension of each of the respective limit regionsto inhibit wicking of fluid from the limit regions into the secondchannel, the first channels and the limit regions containing a polymersolution suitable for electrophoretic sample manipulation and the secondchannel containing a buffer fluid suitable for electroosmotic samplemanipulation.
 2. A microfluidic device as claimed in claim 1, wherein aminimum cross-sectional dimension of each of the limit regions issufficiently smaller than a minimum cross-sectional dimension of thesecond channel such that differential capillary forces prevent fluidfrom wicking from the first channels through the limit regions and intothe second channel when no fluid is present in the second channel.
 3. Amicrofluidic device as claimed in claim 1, wherein the at least onesecond channel has at least one cross-sectional dimension of betweenabout 0.5 and 100 microns.
 4. A microfluidic device as claimed in claim1, wherein the at least one second channel and the plurality of firstchannels each are about 70 microns wide and about 10 microns deep.
 5. Amicrofluidic device as claimed in claim 1, wherein the limit regionseach have a depth of about 2 microns.